1. Field of the Invention
This invention relates to optical indicators and more particularly to apparatus for aiming a gun.
2. Description of the Prior Art
Portable apparatus for accurately aiming a gun has long been sought after by military establishments throughout the world. One aspect of the aiming is causing the barrel of the gun to have a desired angle with respect to a horizontal plane (referred to as an angle of elevation). A desired angle of elevation of the gun is a function of the range of a target. The desired elevation is indicated by one type of apparatus which is known in the art as a heads-up display (referred to as a HUD).
Typically, the HUD includes a beam splitter comprised of a transparent plate having a coating which partially transmits and partially reflects received light. The beam splitter is fixedly connected to the gun in a manner which provides to an operator a view through the beam splitter of a target area.
Additionally connected to the gun is an aiming reticle disposed between a lamp and a collimating lens whereby light, comprising a collimated light image of the reticle, is received by the beam splitter. Because the beam splitter partially reflects light, the image is reflected from the beam splitter to the operator. The collimation of the image causes it to appear to the operator to be distant, although the beam splitter may be a few inches from the operator. Accordingly, the view is of the image superimposed upon the target area.
The reticle is usually comprised of a plurality of straight wire members in a parallel arrangement with equal spacing therebetween whereby the image is comprised of a plurality of equally spaced horizontal lines (referred to as elevation lines).
Typically, one of the elevation lines is identified as being associated with a target by a range finding device whereby the desired elevation is indicated by the identified elevation line. Additionally, when the operator adjusts the elevation of the gun to cause an alignment of the identified elevation line with the target, the gun has the desired elevation.
Because the image is comprised of a plurality of elevation lines, the operator may inadvertently cause an alignment of an unidentified elevation line with the target, thereby causing a projectile from the gun to miss the target. Additionally, it should be appreciated that the range of the target may vary widely. Therefore, it is desirable to have the lowest and highest elevation lines of the image define a large span (on the beam splitter). However, it is difficult to construct a lens for providing the image which defines the large span.